The present invention relates to a thermal transfer image-receiving sheet, and more particularly to a thermal transfer image-receiving sheet that can prevent misregistration which is likely to take place in the formation of a color image on the thermal transfer image-receiving sheet.
In a conventional method for the formation of a color image on a thermal transfer image-receiving sheet, a thermal transfer sheet, comprising colorant layers of three colors of yellow, magenta, and cyan or optionally four colors of yellow, magenta, cyan, and black provided in a face serial manner, and a thermal transfer image-receiving sheet optionally provided with a colorant-receptive layer are first passed through between a heating device and a platen roller pushed by a certain pressure. At that time, a heating device in its heating portion is selectively heated according to image information to allow the colorant contained in the colorant layer in the thermal transfer sheet to migrate into the colorant-receptive layer in the thermal transfer image-receiving sheet to form an image. In the thermal transfer sheet, colorant layers of three colors or four colors are provided in a face serial manner, and different colors are successively transferred three times or four times color by color onto an identical position of the thermal transfer image-receiving sheet to superimpose colors on top of one another to form a color image. In particular, sublimation dye thermal transfer (sublimation-type thermal transfer) is superior to ink melt thermal transfer (hot melt-type thermal transfer) in higher resolution and multi-gradation expression, and is used in applications where high image quality is required.
One method for forming this type of color image is to superimpose individual colors on top of one another in a face serial manner. In this method, since color images are formed screen by screen, while reciprocating the thermal transfer image-receiving sheet, an image is transferred color by color from the thermal transfer sheet successively wound in one direction to superimpose the colors on top of each other or one another. This method is advantageous in that the printing speed is high and, unlike a serial method, there is no overlap between lines. Therefore, if the positional accuracy is high, then high-quality images could be formed. In this method, however, since the thermal transfer image-receiving sheet is reciprocated, the positioning accuracy of the sheet is low. Therefore, the so-called xe2x80x9cmisregistrationxe2x80x9d is likely to occur. Further, for example, it is difficult to realize a reduction in size and weight and a reduction in cost of printers used for this method.
The following several types of printers for this method have hitherto been proposed.
For example, a printer, wherein a thermal transfer image-receiving sheet is reciprocated in such a state that one end of the thermal transfer image-receiving sheet is held by a chuck, has excellent carrying accuracy because the reciprocation is performed by an independent chuck. In this printer, however, although images can be easily printed, for example, on thermal transfer image-receiving sheets of relatively large size, i.e., size A3 or larger size, the mechanism is complicate and the device is large. Further, images cannot be printed on thermal transfer image-receiving sheets of small size without difficulties, and, in addition, the device is expensive. A printer, wherein one end of a thermal transfer image-receiving sheet is fixed and wound on a chuck provided on the surface of a platen roller and the thermal transfer image-receiving sheet is reciprocated by the rotation of the platen roller, has a problem, before the discussion of carrying accuracy, that paper jams are likely to occur at the time of discharge of the thermal transfer image-receiving sheet.
On the other hand, a printer, wherein a thermal transfer image-receiving sheet is held by a grip roller composed of a rubber roller and a metallic roller and is reciprocated by the rotation of the grip roller, is currently most extensively used because, due to simple structure, the size can be reduced and the device is inexpensive. In this printer, the grip roller is composed of a rubber roller for preventing the slip of the sheet and a metallic roller which has on its surface fine projections (hereinafter referred to as xe2x80x9cspikesxe2x80x9d) with a height of about 40 to 100 xcexcm formed by etching, and carries, with high accuracy, the thermal transfer image-receiving sheet while allowing the spikes to bite into the thermal transfer image-receiving sheet. This grip roller has originally been mainly used in single color printing devices, for example, diazo copy of drawings or printers for drafting where the reciprocation of the thermal transfer image-receiving sheet is not required. Therefore, the carrying accuracy of printers using the grip roller is not very good, and the reciprocation of the thermal transfer image-receiving sheet to print an image is likely to cause misregistration. The carrying accuracy at the time of reciprocation can be improved by increasing the pressure for pushing the rubber roller and the metallic roller. In this case, however, the spikes of the metallic roller excessively bite into the thermal transfer image-receiving sheet to leave traces of the spikes. In particular, in the case of thin thermal transfer image-receiving sheets, the traces of spikes become a serious problem, and, in some cases, the traces of spikes reach the image-receiving face in the surface of the thermal transfer image-receiving sheet, leading to lowered quality of color prints.
In order to solve the above problem, thermal transfer image-receiving sheets respectively compatible with various types of printers have hitherto been supplied, for example, in consideration of properties of commercially available printers, for example, carrying accuracy of printers, pushing pressure of the grip roller for improving the carrying accuracy, and the level of traces of spikes caused by increasing the pushing pressure of the grip roller. For example, in order to improve the carrying accuracy, the thickness of the thermal transfer image-receiving sheet has been increased so that, even when the pushing pressure of the grip roller is increased, the influence of the spikes does not reach the image-receiving face.
Supplying thermal transfer image-receiving sheets of a wide variety of specifications, which have been rendered respectively compatible with various printers, however, significantly increases development cost and production cost. When the thickness of the thermal transfer image-receiving sheet is increased from the viewpoint of reducing the influence of the spikes, the thickness and layer construction regarded as suitable for conventional thermal transfer image-receiving sheets are limited and this often limits functions usually possessed by the thermal transfer image-receiving sheets, for example, handle, nerve, gloss and other properties.
The present invention is directed to a solution of the problems of the prior art, and it is an object of the present invention to provide a thermal transfer image-receiving sheet which, even when used with various printers, can be carried with improved accuracy, can form color images without misregistration, and does not permit traces of spikes to reach an image-receiving face and, thus, can minimize the influence of spikes on the quality of images.
The above object of the present invention can be attained by a thermal transfer image-receiving sheet comprising:
a substrate; and
a colorant-receptive layer provided on the substrate,
said substrate being formed of a laminate comprising a plastic film having in its inside microvoids, a support, and a backside film provided in that order from the side of the interface with the colorant-receptive layer, said backside film being an unstretched polyolefin film.
By virtue of the above construction, the thermal transfer image-receiving sheet according to the present invention can be carried in printers with improved accuracy, can form color images without misregistration, and does not permit traces of spikes to reach an image-receiving face and, thus, can minimize the influence of spikes on the quality of images.
According to the present invention, the backside film preferably has a thickness of 12 to 40 xcexcm.
Further, the backside film is preferably formed of straight-chain low-density polyethylene.
Preferably, a polyolefin layer having a thickness of not less than 10 xcexcm is provided between the support and the backside film.